This invention is directed to electronic amplification circuits and, more particularly, to electronic amplification circuits that include an auto-zero mode of operation.
In many electronic circuits, accuracy of amplifier gain is not as important as the accuracy of the relationship between the shape of the amplified waveform and the waveform of the input signal. Contrariwise, in other circuits, accuracy of amplifier gain is of critical importance. One environment wherein electronic amplifier circuits are required to provide exact gain values is in the measurement of the voltage level of unknown signals. More specifically, for various reasons it is often necessary to amplify or buffer the input signal received by electronic measuring systems prior to measuring the voltage level of the signal. In some instances the voltage level of the input signal is too low for it to be accurately measured. In other instances, the input voltage must be decoupled from its source to prevent source loading. In any event, in order to accurately measure such voltages, the gain of the input amplifier must be accurate and remain accurate regardless of amplifier drift. Further, the amplifier must introduce minimal offset voltage and other errors.
In the past, temperature compensated linear amplifiers and chopper-amplifier circuits have been used in measuring circuits to achieve the required gain accuracy. However, these and other similar, relatively complex, amplifier circuits have the disadvantage that they are expensive to produce and lack reliability due to the number of components included in such circuits.
In an attempt to overcome the disadvantages of relatively complex accurate gain amplifier circuits, electronic measurement systems more recently have included auto-zero amplifier circuits. An auto-zero amplifier circuit is a circuit wherein the input of the amplifier is switched between an auto-zero mode of operation and an amplification mode of operation. During the auto-zero mode of operation, the input signal is referenced to a fixed base line value (usually ground) and a capacitor is charged to a voltage level equal to the input offset voltage of the amplifier. (As will be readily appreciated by those skilled in the electronics art, the input offset voltage of the amplifier varies in accordance with drift, i.e., the input offset voltage is sensitive to various parameters, such as temperature, power supply voltage, time, etc.) The capacitor charge is then utilized to counteract the effect of input offset voltage errors during the amplification mode of operation, whereby the input signal is accurately amplified. In other words, auto-zero amplifier circuits do not depend on the accuracy of the amplifier. Rather, such circuits use a relatively inaccurate amplifier, but store a signal related to the inaccuracy of the amplifier. The stored signal is then used to compensate for amplifier inaccuracy during an amplification mode of operation.
While prior art auto-zero amplifier circuits have overcome many of the disadvantages of the expensive, complicated amplifiers utilized in the past, prior art auto-zero amplifier circuits also have disadvantages. Specifically, prior art auto-zero amplifier circuits have had a very limited dynamic range because they have to reference the auto-zero capacitor to ground (or some fixed voltage value) during the auto-zero mode of operation. This arrangement has resulted in the level of the voltages at the input and output terminals of the amplifier continuously swinging between the signal voltage level (during the amplification mode of operation) and ground or a fixed voltage value (during the auto-zero mode of operation). These relatively large voltage swings cause a number of problems. Specifically, large voltage swings create spikes and noise proportional to the magnitude of the common-mode swing, resulting in offset voltage errors, gain errors and noise in the output voltage. In addition, any common mode amplifier errors present in the signal, such as common-mode rejection ratio errors, are not improved because the auto-zero mode of operation is referenced to a fixed level (e.g. ground), which is independent of the input signal level. Further, because a trade-off exists between the large voltage signal swings and fast auto-zero speeds due to the time required for the amplifier to swing between the input signal level and the auto-zero level, and stabilize, limitations are placed on amplifier slew rate and bandwidth. Further, in conventional auto-zero amplifier circuits, switching between the auto-zero mode of operation and the accurate amplification mode of operation is accomplished using junction field effect transistor (JFET) or metal oxide semiconductor field effect transistor (MOSFET) switches. Since the voltage swings occur across the switching devices, the voltage rating of the switching devices limit acceptable voltage swings and, thus, the dynamic range of prior art auto-zero amplification circuits. In this regard, practical prior art auto-zero amplifier circuits have generally been limited to a dynamic range of +2 volts or less. Obviously, it would be desirable to increase the dynamic range of auto-zero amplifier circuits and overcome the foregoing disadvantages without unduly increasing the complexity or cost of such circuits.
Therefore, it is an object of this invention to provide a new and improved auto-zero amplifier circuit.
It is also an object of this invention to provide a new and improved auto-zero amplifier circuit having a wide dynamic range.
It is another object of this invention to provide a new and improved auto-zero amplifier circuit having very little offset voltage error, low gain error and low noise in its output signal.
It is still another object of this invention to provide a new and improved auto-zero amplifier circuit wherein voltage swings between the auto-zero mode of operation and the accurate amplification mode of operation are relatively small.